1. Field of the Invention
This invention relates generally to electromagnetic transducers of the type that may be employed as electro-acoustical drivers for loudspeakers. More particularly, the invention relates to electromagnetic transducers and loudspeakers configured for removing heat via air flow.
2. Related Art
An electro-acoustical transducer may be utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical signals. The basic designs and components of various types of electro-acoustical transducers are well-known and therefore need not be described in detail. An electro-acoustical transducer typically includes mechanical, electromechanical, and magnetic elements to effect the conversion of an electrical input into an acoustical output. For example, the transducer typically includes a magnetic assembly, a voice coil, and a diaphragm. The magnetic assembly and voice coil cooperatively function as an electromagnetic transducer (also referred to as a driver or motor). The magnetic assembly typically includes a magnet (typically a permanent magnet) and associated ferromagnetic components—such as pole pieces, plates, rings, and the like—arranged with cylindrical or annular symmetry about a central axis. By this configuration, the magnetic assembly establishes a magnetic circuit in which most of the magnetic flux is directed into an annular (circular or ring-shaped) air gap (or “magnetic gap”), with the lines of magnetic flux having a significant radial component relative to the axis of symmetry. The voice coil typically is formed by an electrically conductive wire cylindrically wound for a number of turns around a coil former. The coil former and the attached voice coil are inserted into the air gap of the magnetic assembly such that the voice coil is exposed to the static (fixed-polarity) magnetic field established by the magnetic assembly. The voice coil may be connected to an audio amplifier or other source of electrical signals that are to be converted into sound waves. The diaphragm includes a flexible or compliant material that is responsive to a vibrational input. The diaphragm is suspended by one or more supporting elements of the loudspeaker (e.g., a surround, spider, or the like) such that the flexible portion of the diaphragm is permitted to move. The diaphragm is mechanically referenced to the voice coil, typically by being connected directly to the coil former on which the voice coil is supported.
In operation, electrical signals are transmitted as an alternating current (AC) through the voice coil in a direction substantially perpendicular to the direction of the lines of magnetic flux produced by the magnet. The alternating current produces a dynamic magnetic field, the polarity of which flips in accordance with the alternating waveform of the signals fed through the voice coil. Due to the Lorenz force acting on the coil material positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil (or coil former) is attached. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air. Pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level, and are a function of the characteristics of the electrical signals supplied to the voice coil.
Because the material of the voice coil has an electrical resistance, some of the electrical energy flowing through the voice coil is converted to heat energy instead of sound energy. The heat emitted from the voice coil may be transferred to other operative components of the loudspeaker, such as the magnetic assembly and coil former. The generation of resistive heat is disadvantageous for several reasons. First, the conversion of electrical energy to heat energy constitutes a loss in the efficiency of the transducer in performing its intended purpose—that of converting the electrical energy to mechanical energy utilized to produce acoustic signals. Second, excessive heat may damage the components of the loudspeaker and/or degrade the adhesives often employed to attach various components together, and may even cause the loudspeaker to cease functioning. For instance, the materials of certain components themselves, as well as adhesives and electrical interconnects (e.g., contacts, soldered interfaces), may melt or become fouled or otherwise degraded. As additional examples, the voice coil may become detached from the coil former and consequently fall out of proper position relative to other components of the driver, which adversely affects the proper electromagnetic coupling between the voice coil and the magnet assembly and the mechanical coupling between the voice coil and the diaphragm. Also, excessive heat will cause certain magnets to become demagnetized; for example, neodymium (Nd) magnets will demagnetize above about 250° F. Thus, the generation of heat limits the power handling capacity and distortion-free sound volume of loudspeakers as well as their efficiency as electro-acoustical transducers. Such problems are exacerbated when one considers that electrical resistance through a voice coil increases with increasing temperature. That is, the hotter the wire of the voice coil becomes, the higher its electrical resistance becomes and the more heat it generates. As explained below, the problem with heat generation is exacerbated in dual-coil/dual magnetic gap designs.
Due to advantages such as lighter weight and higher power handling, dual-coil/dual magnetic gap designs have been supplanting single-coil designs in loudspeakers. Many dual-coil/dual-gap designs are able to produce more power output per transducer mass and dissipate more heat than conventional single-coil designs. In a dual-coil drive, the voice coil includes two separate windings axially spaced from each other to form two coils, although the same wire may be employed to form both coils. U.S. Pat. No. 5,748,760, commonly assigned to the assignee of the present disclosure, describes an example of a dual-coil design. In U.S. Pat. No. 5,748,760, the magnet assembly includes a stacked arrangement in which a magnet is axially interposed between a front pole piece and a rear pole piece. An outer ring is annularly disposed about the stacked arrangement such that an annular magnetic gap is defined between the outer ring and the stacked arrangement. The two coils are wound around a coil former and inserted into the gap such that one coil is located between the front pole piece and the outer ring and the other coil is located between the rear pole piece and the outer ring, in effect providing two magnetic gaps axially spaced from each other. As both coils provide forces for driving the diaphragm, the power output of the loudspeaker may be increased without significantly increasing size and mass. In addition, the magnet employed in the dual-coil drive is often a neodymium magnet. Neodymium is lighter in weight and provides more magnetic flux per mass (or weight) than more conventional magnetic materials such as ceramics, alnico, and the like. Accordingly, a neodymium magnet can be provided in a smaller size as compared with a more conventional magnet providing the same amount of magnetic flux. The utilization of a neodymium magnet also permits the utilization of smaller pole pieces.
The dual-coil configuration described in U.S. Pat. No. 5,748,760 provides more coil surface area as compared with many single-coil configurations, and thus ostensibly is capable of dissipating a greater amount of heat at a greater rate of heat transfer. For example, a dual coil design that doubles the surface area and number of turns of the coil winding may increase (e.g., nearly double) the capacity of the coil to dissipate heat. However, insofar as a desired advantage of the dual-coil driver is its ability to operate at a greater power output, so operating the dual-coil driver at the higher power output concomitantly causes the dual-coil driver to generate more heat. Hence, the improved heat dissipation inherent in the dual-coil design may be offset by the greater generation of heat. In U.S. Pat. No. 5,748,760, this problem is addressed by configuring the housing such that it contacts both the outer ring and the inner stacked arrangement of the magnetic assembly. As a result, a good amount of surface area is available for transferring heat from the magnetic assembly to the ambient environment via thermal conduction through the material of the housing. In addition, the housing includes cooling fins that further increase the surface area of the housing and consequently further enhance heat transfer. Moreover, the fins are positioned within the loudspeaker such that the fins are in thermal contact with air flowing through the housing as a result of the oscillating diaphragm.
Despite the foregoing approaches toward cooling dual-coil drivers, a need remains for further improvements. As compared to single-coil drivers, adequate heat dissipation in many dual-coil drivers, and more generally multiple-coil drivers, continues to be problem due to the longer thermal paths that must be traversed between the heat source (primarily the voice coil) and the ambient environment. For instance, in many desirable designs for multiple-coil drivers, one or more coils of the driver may be physically located at a significant distance from the ambient environment. Moreover, as noted above, neodymium magnet material or other thermally sensitive magnet material prone to demagnetization is utilized in many popular designs for multiple-coil drivers. In such designs, the neodymium magnet material is often positioned within (radially inside of) the voice coil, where the magnet material rapidly receives a large amount of heat energy due to its proximity to the voice coil and the large thermal gradient established between the magnet material and the voice coil. Accordingly, a need exists for providing improved means for rapidly removing significant amounts of heat from electrically conductive coil structures and magnetic structures during the operation of transducers and transducer-containing devices such as loudspeakers and the like.